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Gl581 Preprint

1.
c ESO 2009
Astronomy & Astrophysics manuscript no. GJ581
April 21, 2009
The HARPS search for southern extra-solar planets
XVIII. An Earth-mass planet in the GJ 581 planetary system
M. Mayor1 , X. Bonﬁls2,3 , T. Forveille2 , X. Delfosse2 , S. Udry1 , J.-L. Bertaux4 , H. Beust2 , F. Bouchy5 , C. Lovis1 ,
F. Pepe1 , C. Perrier2 , D. Queloz1 , and N. C. Santos1,6
1
Observatoire de Gen` ve, Universit´ de Gen` ve, 51 ch. des Maillettes, CH-1290 Sauverny, Switzerland e-mail:
e e e
michel.mayor@obs.unige.ch
2
Laboratoire d’Astrophysique, Observatoire de Grenoble, Universit´ J. Fourier, Uni. J. Fourier, CNRS (UMR5571), BP 53, F-38041
e
Grenoble, Cedex 9, France
3
Centro de Astronomia e Astrof´sica da Universidade de Lisboa, Observat´ rio Astron´ mico de Lisboa, Tapada da Ajuda, 1349-018
ı o o
Lisboa, Portugal
4
Service d’A´ ronomie du CNRS, BP 3, 91371 Verri` res-le-Buisson, France
e e
5
Institut d’Astrophysique de Paris, CNRS, Universit´ Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
e
6
Centro de Astrof´sica, Universidade do Porto, Rua das Estrelas, P4150-762 Porto, Portugal
ı
Received / Accepted
ABSTRACT
The GJ 581 planetary system was already known to harbour three planets, including two presumably rocky planets which straddle its
habitable zone. We report here the detection of an additional planet – GJ 581e – with a minimum mass of 1.9 M⊕ . With a period of
3.15 days, it is the innermost planet of the system and has a ∼5% transit probability.
We also correct our previous confusion of the orbital period of GJ 581d (the outermost planet) with a one-year alias, thanks to
an extended time span and many more measurements. The revised period is 66.8 days, and locates the semi-major axis inside the
habitable zone of the low mass star.
The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination. The planets cannot be more
massive than approximately 1.6 times their minimum mass.
Key words. stars: individual: GJ 581 – stars: planetary systems – stars: late-type – technique: radial-velocity
1. Introduction et al. 2007), GJ 176 (Forveille et al. 2008), and here a fourth
planet in the GJ 581 system. These 6 planets all have mini-
HARPS is a vacuum spectrograph designed to measure precise mum masses under approximately 15 Earth-masses, and repre-
radial velocities, with the speciﬁc goal of searching for exo- sent approximately half of the known inventory of planets orbit-
planets in the Southern hemisphere Mayor et al. (2003). This ing M stars, and most of its lowest mass members. Besides the
high-resolution Echelle spectrograph (R=115000) is ﬁber-fed by HARPS planets, this inventory includes three planets around GJ
the ESO 3.6-meter telescope at La Silla Observatory. The con- 876 (Delfosse et al. 1998; Marcy et al. 1998, 2001; Rivera et al.
sortium which built this instrument was granted 500 observing 2005), a single Neptune-mass planet orbiting GJ 436 (Butler
nights over ﬁve years to conduct a comprehensive search for ex- et al. 2004), jovian planets bound to GJ 849 (Butler et al. 2006)
oplanets. This large programme addresses several key exoplanet and to GJ 317 (Johnson et al. 2007), and ﬁnally a gaseous gi-
questions, including the characterization of very low mass exo- ant planet orbiting GJ 832 (Bailey et al. 2009). Altogether, 8
planets. A signiﬁcant fraction of the observing time was devoted planetary systems bound to M dwarfs have been identiﬁed by
to the study of planets orbiting stars at the bottom of the main the radial velocity technique, for a total of 12 planets. Although
sequence. The M-dwarf sub-programme includes over 100 stars, the statistics are still limited, and come from surveys with diﬀer-
which form a volume-limited sample. ent sensitivities, they indicate that multi-planet systems are com-
Our HARPS search for southern exoplanets orbiting M- mon: the fraction of known multiple systems around M dwarfs
dwarfs is on-going, and has been recently expanded to a larger is already 2/8 (25 %), and it reaches 5/8 (63 %) if one accounts
sample of some 300 low-mass stars. Several planets have al- for systems where a radial velocity drift indicates an additional
ready been detected in the M-dwarf survey: GJ 581 b (Bonﬁls long-period planet.
et al. 2005b), GJ 581 c and d (Udry et al. 2007), GJ 674 (Bonﬁls
The diﬀerent teams which monitor M dwarfs with Doppler
spectroscopy together observe over 300 stars since several years.
Send oﬀprint requests to: M. Mayor
The frequency of planetary systems with at least one identiﬁed
Based on observations made with the HARPS instrument on the
gaseous giant planet orbiting these low mass stars is about 1.5%,
ESO 3.6 m telescope under the GTO program ID 072.C-0488 at Cerro
and clearly smaller than the equivalent quantity for K and G
La Silla (Chile). Our radial-velocity, photometric and Ca II H+K in-
dwarfs. Up to periods of several hundred days this comparative
dex time series are available in electronic format the CDS via anony-
deﬁcit of Jupiter-mass planets for M dwarfs is statistically ro-
mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-
bust (Bonﬁls et al. 2006; Johnson et al. 2007). Conversely, plan-
strabg.fr/cgi-bin/qcat?J/A+A/

2.
2 M. Mayor et al.: An Earth-type planet in GJ 581 planetary system
ets less massive than ∼25 M⊕ are signiﬁcantly more frequent 1570 days (4.3 years). In parallel to this extension of the data set,
around M dwarfs (Bonﬁls et al. 2007). the precision of all measurements, including the previously pub-
The GJ 581 system is of particular interest, as two of its pre- lished ones, has been improved. The reference wavelengths of
viously detected planets are located on the two edges of the hab- the thorium and argon lines of the calibration lamps were re-
itable zone (HZ) of the M3V host star (Selsis et al. 2007; von vised by Lovis & Pepe (2007), and most recently Lovis (2009,
Bloh et al. 2007). Despite the uncertainties on the exact loca- in prep.) implemented a correction for the small eﬀect of Th-Ar
tion of the “liquid water” zone for planets more massive than the calibration lamp aging. The changing internal pressure of aging
Earth, these detections demonstrate that rocky planets in the HZ lamps shifts the wavelength of their atomic transition. The ef-
can be detected by Doppler spectroscopy, for planets orbiting M fect is now corrected down to a level of just a few 0.1 m/s, using
dwarfs. the diﬀerential pressure sensitivity of the Argon and Thorium
In this paper we report the detection in the GJ 581 system lines. The combined photon noise and calibration error reult in
a typical overall uncertainty of 1 m s−1 , with a full range of 0.7
of a fourth planet, ’e’, with a minimum mass of 1.94 M⊕ . We
to 2 m s−1 which reﬂects the weather conditions. The pipeline
also correct the period of the outer planet GJ 581d, and we ﬁnd
that the revised period locates GJ 581d in the habitable zone. In also shifts the radial velocities to the Solar System Barycentric
section 2 we brieﬂy recall the main characteristics of the host reference frame (Lindegren & Dravins 2003), and automatically
star GJ 581, and of its ﬁrst three planets. An enlarged set of subtract the RV drift due to perspective acceleration from the
measurements (section 3) allows a reexamination of the struc- proper motion of the star (K¨ rster et al. 2003); 20 cm/s/yr for GJ
u
ture of the GJ581 planetary system, and the discovery of an ad- 581.
ditional very-low-mass planet (section 4). Since Doppler spec-
troscopy, and all the more so for M dwarfs, can be confused
4. GJ 581, a system with four low-mass planets
by stellar surface inhomogeneities, we dedicate section 5 to a
discussion of GJ 581’s magnetic activity. In section 6, dynami- Our preferred method for planet searches in RV data uses a
cal stability considerations allow us to obtain an upper limit to heuristic algorithm, which mixes standard non-linear minimiza-
the planetary masses. To conclude, we brieﬂy discuss how this tions with genetic algorithms. It eﬃciently explores the large pa-
system meshes up with theoretical predictions, and consider the rameter space of multi-planet systems, and quickly converges
prospects of ﬁnding even lower mass planets in the habitable toward the best solution. We tried models composed of zero to
zone of M dwarfs. ﬁve planets on Keplerian orbits, and found that the data require
a 4-planets model.
To verify the robustness of that ’black-box’ solution and gain
2. Stellar caracteristics and planetary system
insight on its content, we also performed a step by step peri-
Our report of the ﬁrst Neptune-mass planet on a 5.36-d orbit odogram analysis. The top panel of Fig. 1 displays the window
around GJ 581 (Bonﬁls et al. 2005b) extensively describes the function of our RV measurements, and unsurprisingly shows that
properties of the host star. Here we will therefore only summa- the dominant periodicities of our sampling are 1 day and 1 year.
rize those characteristics. The second panel is a ﬂoating-mean peridogram (Gilliland &
GJ 581 (HIP 74995, LHS 394) is an M3 dwarf at a distance Baliunas 1987; Zechmeister & K¨ rster 2009) of the RVs. It is
u
to the Sun of 6.3 pc. Its estimated luminosity is only 0.013 L . dominated by a ∼5.36-day periodicity, which corresponds to
Bonﬁls et al. (2005a) ﬁnd GJ 581 slightly metal-poor ([Fe/H] = GJ 581b. To estimate the FAP of that peak, we generated vir-
-0.25), in contrast to the supersolar metallicities (Santos et al. tual data sets using boostrap resampling (Press et al. 1992) of
2001, 2004) of most solar-type stars hosting giant planets. All the actual measurements, and examined the peak power in the
indirect tracers (kinematics, stellar rotation, X-ray luminosity, periodogram of each of these virtual datasets. None of 10,000
and chromospheric activity) suggest a minimum age of 2 Gyr. trials had as high a peak as measured for the actual 5.36-day
The HARPS spectra show weak Ca  H and K emission, in the signal, which therefore has a FAP below 0.01%. We then ad-
lower quartile of stars with similar spectral types. This very weak justed a Keplerian orbit with that starting period, and exam-
chromospheric emission points towards a long stellar rotation ined the periodogram of the residuals of this 1-planet solution
(Fig. 1, 3rd panel). That second periodogran is dominated by
period.
The ﬁrst planet was easily detected after only 20 HARPS a peak around ∼ 12.9 day, with a FAP<0.01%, which corre-
observations (Bonﬁls et al. 2005b), and the periodogram of the sponds to the known planet GJ 581c. We then investigated the
residuals of that ﬁrst solution hinted at power excess around periodogram of the residuals of the corresponding 2-planets least
13 days. As 30 additional observations were accumulated for a square adjustment. That new periodogram has power excess in
total time span of 1050 days, that coherent signal strengthened the 50-90 days range and around 3.15 days (as well as at 1-
to strong signiﬁcance, and a third planet appeared, though still day aliases of these periods), both with FAPs<0.01%. The broad
with a signiﬁcant False Alarm Probability (FAP): GJ 581c had power excess between 50 and 90 days splits into 3 separate peaks
m sin i = 5M⊕ , P = 12.9 days) and a ∼ 0.28% FAP, while the at ∼59, 67 and 82 days, which are 1-year aliases of each other
more distant GJ 581d had m sin i = 7.7M⊕ , P = 83 days, and a (1/67 − 1/365 ∼ 1/82, 1/67 + 1/365 ∼ 1/59, ). In Udry et al.
∼ 3% FAP. (Udry et al. 2007). Udry et al. (2007) therefore ended (2007), we attributed an 82-days period to GJ 581 d. The corre-
on a call for conﬁrmation by additional radial velocity measure- sponding periodogram peak now has markedly less power than
ments, and based on the very-low magnetic activity of GJ 581, the 67-days periodicity, and attempts to adjust orbits with that
suggested that the signiﬁcant residuals might reﬂect additional period (as well as with a 59 days period) produced signiﬁcantly
planets in the system. larger residuals. We conclude that the actual period of GJ 581
d is around 67 days and that we had previously been confused
by a 1-year alias of that period. As we discuss below, the re-
3. A new data set
vised period locates GJ 581 d in the habitable zone of GJ 581.
Following up on that call for more data, we have now recorded a Subtracting the corresponding 3-planet adjustement leaves no
power excess in the 50-90 days range (5th panel), and markedly
total of 119 H measurements of GJ 581, which together span

3.
M. Mayor et al.: An Earth-type planet in GJ 581 planetary system 3
1
enhances the 3.15 day peak. The residuals of this 4-planet so-
lution ﬁnally display no statistically signiﬁcant peak (6th panel).
Window
The strongest peak is located close to one year (P ∼ 384 day) 0.5
and has a ∼50% FAP.
Both the heuristic algorithm and the spectral analysis method
ﬁnd the same 4 coherent signals in our data. Once this best 0
approximate 4-planet solution had been identiﬁed, we per-
500
formed a ﬁnal global least square adjustment, using the iterative
Levenberg-Marquardt algorithm. Since we ﬁnd insigniﬁcant ec-
centricities for planets ’b’ and ’c’, we ﬁxed the eccentricities and 250
longitude of periastron to zero for both planets. Table 1 lists the
resulting orbital elements, and Fig. 2 displays the corresponding
Keplerian orbits. 0
Compared to our Udry et al. (2007) study, we thus add a
100
fourth planet and very signiﬁcantly revise the characteristics of
the outer planet (GJ 581d), and in particular its period. Planets
b to e have respective minimum masses of m sin i ∼ 15.7, 5.4, 50
7.1 and 1.9 M⊕ . GJ 581e is the lowest mass exoplanet detected
so far around a main sequence star. None of the period ratios
of the system comes close to any integer value, with e.g. Pb /Pe 0
= 1.70, Pc /Pb = 2.4 and Pd /Pc = 5.17. Non-resonant hierarchies
40
have similarly been noticed for all multi-planet systems which
only contain low mass planets, such as HD 40307 (Mayor et al.
Power
2009) and HD 69830 (Lovis et al. 2006). 20
5. Planets rather than spots 0
Coherent Doppler shifts unfortunately do not always correspond 50
to planets. Inhomogeneities of the stellar surface like spots,
plages, ﬂares or convective patterns can break the even distri-
25
bution between the blue- and red-shifted halves of a rotating star
and induce apparent radial velocity shifts. At best, this intro-
duces noise on the RV measurements, which is usually referred
0
to as “jitter”. In some cases the surface pattern can be stable over
long time spans (from days to years), and the “jitter” noise then 10
is correlated. It then does not average out as white noise would,
and instead builds up a coherent signal. That apparent Doppler
5
shift induced can easily happen to mimic a Keplerian orbit (e.g.
Queloz et al. 2001; Bonﬁls et al. 2007).
M dwarfs notoriously tend to have stronger magnetic activ-
0
ity than solar-like stars, and possible false detections from co-
1 10 100 1000
herent jitter therefore need even closer scrutiny. We have indeed
Period [day]
encountered several such cases in our M star survey, including
GJ 674 (Bonﬁls et al. 2007) and GJ 176 (Forveille et al. 2008).
Fig. 1. Periodograms of GJ581 radial velocity measurements. Panel
In both cases, photometric monitoring and spectroscopic activ-
1 (top): Window function of the HARPS measurements. Panel 2:
ity diagnostic (measuring the Ca  H&K and Hα lines) demon-
Periodogram of the HARPS velocities. The peaks corresponding to
strated that a seeemingly Keplerian signal was in fact due to GJ581b (P=5.36d) and its alias with the sideral day are clearly visible.
spot modulation of the rotational proﬁle. These two examples Panel 3: HARPS velocities corrected from the velocity variation due to
serve as reminders that a periodic Doppler signal does not al- GJ581b. The peak corresponding to GJ581c (P=12.9d) appears. Panel
ways reﬂect the presence of a planet, and that additional checks 4: Removing the variability due to planets b and c, we can see the peak
are needed to conclude. due to the P=3.15d variability of GJ581e (+ the alias with the a one
sideral day period) as well as the peak at 66d due to GJ581d. Panel 5 :
In our previous papers on the GJ 581 system, we could
Periodogram after removing the eﬀect of planets b, c and d. The peak
exclude any such confusion for the ﬁrst three planets. The ∼
due to planet e (Mass = 1.9 earth-mass) is evident (+ its alias with the
13m s−1 radial velocity amplitude due to GJ 581 b (P = 5.36
sideral period). Panel 6 (bottom): Removing the velocity wobbles due
day – Bonﬁls et al. 2005b) is suﬃciently large that a spot sig-
to the four planets, the periodogram of the residual velocities shows no
nal of that magnitude would have induced measurable distor-
additional signiﬁcant peaks.
tions of the rotational proﬁle. The lack of any correlation be-
tween the bisector span (a ﬁrst order measurement of spectral
lines asymmetry) and the measured RVs, excludes that scenario.
an unprobably low inclination i to be compatible with a 5 days
The strength of the Ca  H&K emission lines in addition classi-
rotation period, The additional measurements since Bonﬁls et al.
ﬁes GJ 581 in the lowest ∼ 5% magnetic activity bin of our M
(2005b) have strengthened those conclusions even further.
dwarfs sample. This implies a much longer rotation period than
the observed 5.4 days, as conﬁrmed by the very low projected In Udry et al. (2007), we used distinct lines of arguments for
rotational velocity v sin i < 1 km s−1 ). That velocity would need the 12.9 and the 83 days (now 67 days) signals. The arguments

4.
4 M. Mayor et al.: An Earth-type planet in GJ 581 planetary system
Table 1. Fitted orbital solution for the GJ 581 planetary system: 4 Keplerians. The model imposes circular orbits for planets GJ 581 b & e, since
the derived eccentricities for a full Keplerian solution are insigniﬁcant (see text).
Parameter GJ 581 e GJ 581 b GJ 581 c GJ 581 d
3.14942 ± 0.00045 5.36874 ± 0.00019 12.9292 ± 0.0047 66.80 ± 0.14
[days]
P
0.17 ± 0.07 0.38 ± 0.09
0 (ﬁxed) 0 (ﬁxed)
e
ω -110 ± 25 -33 ± 15
[deg] 0 (ﬁxed) 0 (ﬁxed)
[m s−1 ] 1.85 ± 0.23 12.48 ± 0.23 3.24 ± 0.24 2.63 ± 0.32
K
[km s−1 ] -9.2082 ± 0.0002
V
[10−14 M ]
f (m) 0.21 108.11 4.34 10.05
m2 sin i [M⊕ ] 1.94 15.65 5.36 7.09
[AU] 0.03 0.04 0.07 0.22
a
119
Nmeas
[days] 1570
Span
σ (O-C) [ms−1 ] 1.53
χ2 1.49
red
developed above for GJ 581b hold for the 12.9-day period, which measurements as a function of time (top panel), and the corre-
is also too short to be compatible with the rotation period. They sponding periodogram (lower panel). It suggests long term varia-
also apply for GJ 581e at 3.1 days. An 83 days (or 67 days) tions, over timescales of several thousands days which are much
period, on the other hand, would be perfectly consistent with the too long to be the rotational period. If those variation are real,
rotation period of an M dwarf with the activity level of GJ 581. they would be more likely to reﬂect an analog of 11 yers so-
For such long periods the bisector span also loses its diagnostic lar magnetic cycle. Magnetic cycles can potentially aﬀect appar-
power, because the rotational proﬁle becomes much too narrow ent stellar velocities, through their impact on stellar convection
to resolve with HARPS. This calls for a diﬀerent argument, and which can change the balance between ascending (blue-shifted)
we used a relation by Saar & Donahue (1997) to estimate the and receding (red-shifted) material, but we see no evidence for
minimum ﬁlling factor of a spot that could produce the observed long-term RV variations. The other signiﬁcant structure in the
radial velocity signal. That relation, chromospheric signal is a broad power excess between 80 and
120 days, which is much more likely to reﬂect the rotation pe-
K s ∼ 6.5 × f s0.9 × v sin i [m s−1 ] (1) riod. That peak overlapped our incorrect initial estimate of the
period of GJ 581 d, but as discussed above the photometry limits
connects the semi-amplitude K s of the radial-velocity signal
the amplitude of any velocity signal from spots at that period to
induced by an equatorial spot with its surface ﬁlling factor,
at most a quarter of the observed value. The revised period puts
f s %, and the projected rotation velocity, v sin i. It has been vali-
a ﬁnal nail in the coﬃn of that concern, since the periodogram
dated in numerous examples, including e.g. GJ 674 and GJ 176.
of the Ca  H+K indices has no power excess around 66 days.
Applying that relation to a R = 0.31 R stellar radius, a P = 83
To summarize, none of the 4 periodic signal detected in our
days period and a K ∼ 2.5 m s−1 semi-amplitude readily showed
RV measurements is compatible with rotationasl modulation of
that the GJ 581 d signal would have needed a f s ∼ 2.2% ﬁll-
stellar surface patterns. Four low-mass exoplanets orbiting the
ing factor to be explained by a spot. Such a large spot is ﬁrmly
star remain, by far, the most likely interpretation.
excluded by the sub-% photometric stability of GJ 581.
Our revised 66.7 days period for GJ 581 d weakens that ar-
gument slightly, since a shorter period signal can be induced by a
6. Dynamical evolution
smaller spot. The revision however is minor, as the 67-day signal
still requires a f s = 1.7% spot, which is easily excluded by the
We reinvestigate the dynamics of the revised GJ 581 system, us-
improved photometric measurements that have become avail-
ing the same techniques as in Beust et al. (2008, B08). With the
able since Udry et al. (2007). The strongest photometric stability
SyMBA symplectic N-body code (Duncan et al. 1998), we carry
constraint is provided by 6 weeks of continuous monitoring of
5-body integrations over 0.1 Gyr, with a 2 × 10−4 yr timestep.
GJ 581 by the MOST satellite. Preliminary results, presented
Starting from the nominal solutions (Tab.1), which assumes ini-
by J. Matthews at the 2007 Michelson Summer School1 , indi-
tially circular orbits for Gl581e and Gl581b (the two innermost
cate that the peak-to-peak photometric variability during those
planets), we performe thoses integrations for inclinations of 90
6 weeks was less than 5 mmag. The 66.7-day HARPS radial
to 10 degrees, assuming full coplanarity of all orbits.
velocity signal is therefore incompatible with rotational modu-
Figure 4 shows the maximum variations of the semi-major
lation of spots or granulation patches.
axes and eccentricities during those integrations. As in B08, we
Variations of the chromospheric Ca  H+K line provide an-
note that the semi-major variations gradually increase as the in-
other diagnostic of magnetic activity, which proved very val-
clination decreases. This is a clear indication that lower inclina-
ubale for GJ 674 (Bonﬁls et al. 2007). We parametrize this chro-
tions, i.e. higher planetary masses, produce a less stable system.
mospoheric emission by
Compared to our previous study, the addition of GJ 581e drasti-
H+K cally tightens the minimum inclination constraint. That low mass
Index = , (2)
R+B planet has little eﬀect on the stability of the more massive GJ
581b, GJ 581c, and GJ 581d, but for inclinations 5 ≤ i ≤ 30◦ ,
where H (resp. K) is the ﬂux measured in the Ca  H (resp. K)
GJ 581e is ejected out of the system on time scales of at most a
line and where R and B measure the ﬂux in pseudo-continuum
few Myrs.
bands on both sides of the lines. Fig. 3 displays our Ca  H+K
The nominal integrations assume circular initial orbits for GJ
1
581e and GJ581b. Relaxing this constraint and setting the initial
http://nexsci.caltech.edu/workshop/2007/Matthews.pdf

5.
M. Mayor et al.: An Earth-type planet in GJ 581 planetary system 5
Fig. 4. Stability of various conﬁgurations of the four-planet Gl 581 system. The left and right panels respectively show the maximum variations,
after ×108 years, of the semi-major axes and of the eccentricities (right plot) as a function of the assumed inclination of the system (i=0 is pole-on).
Each cross represents one simulation. The grey shading for i < 35◦ marks an instable zone where one or more planets were ejected during the
integration.
eccentricity of planet e to 0.1, we found the system even less tected around main sequence stars. Such a low mass is almost
stable: GJ 581e was then ejected for inclinations up to i = 40◦ . certainly rocky, and its equilibrium temperature is too high to
allow a substantial atmosphere.
Unlike most multi-planet radial-velocity systems, the nomi-
nal least square adjustment to the radial velocities is dynamically GJ 581 has the largest number of measurements in our
stable for suﬃciently high inclinations (and co-planar orbits). HARPS M-star sample, which evidently help detect such a low
Restricting the solutions to dynamically stable systems only im- mass planet in a four planet system. The detection however does
poses that i > 40◦ and therefore that the mass of each planet be
∼ not stretch the limits of Doppler spectroscopy, and for the same
no more than ∼1.6 times its minimum mass. For GJ 581e, b, c period a planet only half as massive would have been detectable
and d, those upper limits are 3.1, 30.4, 10.4 and 13.8 M⊕ . in our dataset. Our HARPS measurements set an upper limit of
1.2 m/s on the stellar jitter of GJ 581, based on a quadratic sub-
traction of just the mean photon noise from the observed dis-
7. Summary and discussion persion of the radial-velocity residuals to the 4-planet solution.
Some small fraction of this value could still be instrumental, but
The large number of precise HARPS measurements has now re-
a 1.2 m/s “jitter” allows the detection of a 2 M⊕ planet with a 25
vealed four planets in a non resonnant conﬁguration. The false
days orbital period. Such a planet would be earth-type and in the
alarm probablities of all four planetary signals are below 0.01%.
middle of the habitable zone of its M dwarf.
The orbital elements of planets GJ581b (P=5.36d Bonﬁls et al.
2005a) and GJ581c Udry et al. (P=12.9d 2007) are fully con- The stellar “jitter” has diﬀerent physical components, with
ﬁrmed, and slightly reﬁned. The 82 days Udry et al. (2007) distinct time scales. For more massive stars and shorter integra-
period for GJ 581d, on the other hand, was a one-year alias tion times, acoustic modes represent a major contribution, but
of the true 66.8 days period of that planet. Our extended span the short acoustic timescales in this small star and the 15 min-
and much larger number of measurements correct that confu- utes integration ensure that they have been averaged out to below
sion. Monitoring of the Ca  H+K activity indicator over the four a few 0.1 m/s. The 1.2 m/s jitter therefore mostly reﬂects longer
years of our spectroscopic measurements indicates that the sig- time-scale phenomena, such as granulation noise and changes
nal at 66 days is not the result of the time variable stellar activity. in stellar surface anisotropies during a magnetic cycle. We are
At the present time, based on the well deﬁned radial velocity sig- developing optimized measurement strategies to average out the
nal and the estimation of the possible eﬀect of the stellar activity granulation noise contribution to the stellar jitter, but HARPS al-
(with the constraint of the small observed photometric variabil- ready demonstrates that the global jitter (including instrumental
ity) the presence of the planet with a period of 66.8 days is the errors, but not photon noise) of reasonably chromospherically
most probable interpretation. quiet M dwarfs is at most of the order of 1 m/s.
The new orbital elements derived for the outer planet GJ This new low mass planet adds to the small set of Doppler-
581d correspond to a 0.22 AU semi-major axis. A planet with detected super-Earths, planets with a minimum mass under
an eccentric orbit receives a larger orbit-averaged ﬂux than a 10 M⊕ . This planetary mass domain, as well as the Neptune-
planet on a circular orbit of the same semi-major axis (Williams mass range (<30 M⊕ ) has largely been populated by the HARPS
& Pollard 2002). GJ 581 d receives the same average ﬂux as a surveys. A few statistical properties have already emerged from
planet on a circular orbit at a = 0.20 AU. The Udry et al. (2007) these early discoveries (Mayor & Udry 2008):
orbital elements of GJ 581d, with a 0.25 AU semi-major axis,
located the planet close to the outer edge of the habitable zone,
− The full distribution of planetary masses is bimodal, with
but most likely just slightly outside (e.g. Selsis et al. 2007). The
revised parameters locate GJ 581d ﬁrmly in the habitable zone. distinct peaks corresponding to gaseous giants and super-
Thanks to 119 precise radial velocity measurements, we have Earths. Despite the observational bias against low mass plan-
ets, the distribution below ∼30 M⊕ rises towards super-Earth
discovered a fourth planets in the GJ 581 system. Its minimum
mass, m sin i = 1.9M⊕ , is by far the lowest amongst planets de- planets (cf. Fig. 7 Mayor & Udry 2008).